1. Field of the Invention
The present invention relates to a method of making transparent laminated windows with a minimum of opacity problems that may result from adhering polyurethane interlayer sheeting to acrylic plastic or another sheet of polyurethane that constitute components of said transparent windows, and a primer composition useful in said method. The present invention is especially concerned with a method of adhering a polymerized preformed sheet of polyurethane that is solid in appearance and difficult to adhere to acrylic plastic or another sheet of polyurethane to a rigid sheet of acrylic plastic or polyurethane during the fabrication of a transparent laminated window.
Laminated windows for aircraft contain alternate layers of rigid transparent material, which may be glass or any well recognized plastic substitute for glass such as polycarbonate compositions, acrylic compositions, (either stretched acrylic or as cast acrylic resins), or polyester resins, alternating with layers of flexible material, such as polyvinyl butyral or polyurethane. When the relatively rigid material is acrylic resin and the relatively flexible material is polyurethane, considerable difficulty is encountered in obtaining a suitable bond to adhere the layers to one another. This is especially so when the polyurethane is a preformed sheet of a polyester type of polyurethane.
Details of polyurethane compositions of the polyester type that have superior properties that make them most suitable for use in windows for aircraft and their method of manufacture for use in aircraft window fabrication may be obtained by reference to South African Patent Specification No. 72/4156 of Nelson V. Seeger and Andrew J. Kaman, the disclosure of which is incorporated herein by reference. Difficulty has been met in the development of an appearance of opacity that spoils the optical properties of transparent windows when various well known primers for polyurethane are used to adhere these polyester type polyurethanes to acrylic plastic sheets or other sheets of polyurethane prior to the present invention.
2. Description of the Prior Art
U.S. Pat. No. 3,657,057 to Norman Shorr and Harry E. Littell, Jr. uses an A-stage liquid polyurethane alone or mixed with organic solvents as a primer for adhering a cured sheet of polyurethane to a rigid transparent sheet such as glass or polycarbonate. When the assembly containing the A-stage liquid polyurethane is subjected to heat and pressure, the A-stage polyurethane polymerizes into the C-stage to produce an improved adhesive bond between the cured polyurethane sheet and glass or polycarbonate. While an A-stage polyurethane primer or a primer solution containing an A-stage polyurethane dissolved in a suitable solvent provides excellent adhesion when the primer is applied shortly after its manufacture, unfortunately such primers have a relatively short shelf life and can only be stored for a very limited period before the polyurethane polymerizes into the less soluble B and C stages or into a solid stage in which adhesion is difficult to attain.
U.S. Pat. No. 3,804,810 to Michael Fryd uses organic solvents, such as chloroform and methylene chloride, in the reaction between a copolyester and a diisocyanate on the one hand and a polyester urethane cross-linker in the formation of a polyester urethane composition.
U.S. Pat. No. 3,428,609 to Kenneth William Chilvers and Gordon Trappe refers to the problem of dissolving solutions containing 10 percent to 80 percent of polyurethane. Both this patent and U.S. Pat. No. 3,503,934 to said Kenneth William Chilvers disclose processes which involves a chemical reaction step for dissolving polyurethane in an inert solvent.
Prior to the present invention, the best results in adhering polyurethane preformed sheets to acrylic sheets were obtained using a primer solution consisting essentially of a copolymer of alkyl methacrylate, alkyl acrylate and an unsaturated acid dissolved in a mixture of ethanol and ethylene dichloride. The present invention has resulted in adhesion values ranging from 5 to 7 times the adhesion value obtained from said prior art composition.